o_19octlt3v29m1ul81r9513q81i4ra.pdf

276 Chapter 15
Note A compression algorithm works by identifying repeated characters or patterns in the data
file and substituting more compact codes for them. For example, an algorithm might change
00000000000000000000 to something like 20*0.
Further, video clips vary according to the number of frames per second (fps); more
frames per second means smoother playback and larger file size. A VHS videotape
records at 30 fps, but for Web use, a frame rate of 15 fps works well because it results in
a much smaller file size. You can set the number of frames per second when you record
the video clip, or use a third-party program to decrease the frames per second of a prerecorded
clip.
When a sound clip is digitized (converted to digital format), a series of sound “snapshots”
are taken per second. These snapshots are called samples. Higher sampling rates (the
number of samples per second) yield higher sound reproduction accuracy, but at the cost
of larger file sizes. Sampling rates for audio clips are measured in kilohertz—for example,
11 KHz, 22K Hz, or 44 KHz.
Note “Kilo” means thousand; an 11 KHz clip contains approximately 11,000 samples per
second.
Sound clips also have varying sample resolutions, which are the number of bits used to
describe each sample. Common sample resolutions are 8-bit, 16-bit, and 32-bit. The
more bits that are sampled, the larger the file will be.
Sound clips can be recorded in either mono or stereo, referring to the number of audio
channels in the recording. Mono uses a single channel, which is duplicated in each
speaker. Stereo uses two channels, with one channel playing back in each of two speakers.
Stereo clips are approximately double the file size of mono ones.
When recording audio clips, you can usually choose between various sampling rates and
resolutions. Here are some of the most common combinations of settings:
Settings
Quality
8 KHz, 8-bit, mono Telephone quality
22 MHz, 16-bit, stereo Radio quality
44 KHz, 16-bit, stereo CD quality
Encoding Video
Now that you have a high-level view of video and audio playback on the Web, you might
be wondering how you encode your favorite vacation videos into three formats (four if
you count Flash). The clips provided for the exercises in this chapter are ready to go, but
you will need to prepare your own video clips on your own.